1. Field of the Invention
The present invention relates to a power supply system switching device which selects one of plural different AC sources by switching when power is supplied to a load from the plural different AC sources, whereby the power is supplied to the load, and more particularly, to a power supply system switching device which can switch the load to connect to an other normal power supply system when one of the sources supplying power to the load is voltage-disturbed, and a method of switching between power supply systems.
2. Description of the Related Art
Related Art 1
FIG. 13 shows a two line normal service/auxiliary switching circuit which is one of the conventional power supply system switching devices, described in Dictionary of Electric Equipment New Edition, published by Industrial Research Center of Japan Inc., May 1989. In FIG. 13, shown are a first AC source 1, which is a first power supply system, a second AC source 2, which is a second power supply system, a first mechanical switch 3, a second mechanical switch 4, and a load 5.
Under normal operation of the first AC source 1, power is supplied from the first AC source 1 to the load 5 while the first switch 3 is dosed, and the second switch 4 is open. In the event that the first AC source 1 is service-interrupted, the continuous power supply ceases. Accordingly, after the first switch 3 is opened, the switch 4 is closed, whereby power is supplied from the second AC source 2 to the load 5 via the switch 4. Thus, power is continuously supplied to the load 5.
However, since the first switch 3 and the second switch 4 are mechanical switches, it takes more than fifty or sixty ms (milliseconds) for the switching. Accordingly, problematically, when the load is circuit-changed, interruption can not be avoided, and undesirable effects on the load can not be prevented.
For higher speed, a switch of which the operation mechanism uses a spring has been developed. However, the opening operation can be performed at a higher speed, but the speed of the closing operation can not be enhanced. Problematically, the switching time as a whole of several cycles can not be avoided.
Related Art 2
FIG. 14 shows another conventional power supply system switching device which is described in xe2x80x9c15 kV, Medium Voltage Static Transfer Switchxe2x80x9d (IEEE, 1995, May/June). This device is a power supply system switching device using a thyristor switch instead of a mechanical switch in order to secure high speed switching between the power supply systems. FIG. 14 shows a first thyristor switch 6 comprising a pair of thryristors connected in inverse-parallel to each other, and a second thyristor switch 7 comprising a pair of thyristors connected in inverse-parallel to each other.
Under normal operation of the first AC source 1, gate signals are continuously given to the thyristor pair of the first thyristor switch 6, so that the thyristor switch 6 is kept closed, while the other hand, no gate signals are given to the thyristor pair of the second thyristor switch 7, so that the second thyristor switch 7 is kept open. Thereby, power is supplied to the load 5 from the first AC source 1 via the first thyristor switch 6.
In the event that the first AC source 1 is service-interrupted, the application of the gate signals to the thyristor pair of the first thyristor switch 6 immediately ceases. After the first thyristor switch 6 is opened at the time when the current flowing in the thyristor reaches the zero point, gate signals to the thyristor pair of the second thyristor switch 7 are applied close that second thyristor to switch 7. Thereby, power is supplied from the second AC source 2 to the load 5 via the second thyristor switch 7.
When such a thyristor switch as described above is used, switching from the first AC source 1 to the second AC source 2 can be performed within a half cycle. Therefore, characteristically, at service-interruption of the first AC source 1, switching to the second AC source 2 can be made without undesirably affecting the load 5.
However, in the above-described system, load current invariably flows through the thyristors. Therefore, there arises the problem that the power loss caused by the internal loss of the thyristors is increased, and the operation cost becomes high due to the generated power loss. Further, problematically, a large-sized cooling device is required since the thyristor elements have to be quenched. Thus, there arises the problem that the switching device has a large-size, and is expensive.
As regards the conventional devices of these types, as described above, those provided with mechanical switches take much time to make a transfer. On the other hand, devices provided with thyristor switches are problematic with regard to power loss, heating, and so forth.
Accordingly, it is an object of the present invention to solve the above-described problems and provide a power supply system switching device which is compact in size and is inexpensive, and can switch from a power supply system to another power supply system without service interruption in the event that the power supply system is disturbed, of which the loss to be generated under conduction can be considerably reduced, significantly decreasing the operation cost, and in which in switching of two different power supply systems by use of a thyristor switch, one thyristor switch can be used in common to switch from one power supply system to the other power supply system and moreover, to switch in the opposite direction, and a method of switching between power supply systems.
According to the present invention, there is provided a power supply system switching device which comprises a first mechanical bypass switch connected in series with a first power supply system, a second mechanical bypass switch connected in series with a second power supply system, a semiconductor switch connected in parallel to the second bypass switch, a load connected to the connection point of the first bypass switch, the second bypass switch, and the semiconductor switch, and a controller for on-off controlling the two bypass switches and the semiconductor switch so that either one of the two power supply systems is selected to supply power to the load by operation of switching between the two bypass switches, the controller, when the power is switched from the first power supply source to the second power supply source, causing the first bypass switch to open and the semiconductor switch connected in parallel to the first bypass switch to conduct whereby the current flowing through the first bypass switch is commutated so as to flow through the semiconductor switch, and thereafter, causing the second bypass switch to close, whereby the load is switched so as to be connected to the second power supply system.
Preferably, in the power supply system switching device of the present invention, the semiconductor switch comprises a pair of semiconductor switches connected in inverse-parallel to each other, and the controller comprises a bypass switch opening section for opening the first bypass switch, a current/voltage detection section for detecting the direction or voltage-polarity of current flowing through the first bypass switch, a first turning-on section for turning on one of the switch elements of the semiconductor switch based on the detection results obtained by the current/voltage detection section, a second turning-on section for turning on both of the switch elements of the semiconductor switch a predetermined period later than the time when the current flowing at the first bypass switch becomes zero, a bypass switch closing-section for closing the second bypass switch a predetermined period later than the time when both of the switch elements of the semiconductor switch are turned on, and a turning-off section for turning off both of the switch elements of the semiconductor switch a predetermined period later than the time when the second bypass switch is closed.
Also preferably, the power supply system switching device for switching between the first and second power supply systems in both directions further comprises a circuit-changing switch for circuit-changing the semiconductor switch to connect selectively in parallel to one of the first and second bypass switches, wherein the semiconductor switch comprises a pair of semiconductor switch elements connected in inverse-parallel to each other, the controller comprises a bypass switch opening section for opening one of the first and second bypass switches, a current/voltage detection section for detecting the directions or voltage-polarities of currents flowing through the first bypass switch and the second bypass switch, a first turning-on section for turning on one of the switch elements of the semiconductor switch based on the detection results obtained by the current/voltage detection section, a second turning-on section for turning on both of the switch elements of the semiconductor switch a predetermined period later than the time when the current flowing through the one bypass switch becomes zero, a bypass switch closing-section for closing the other bypass switch a predetermined period later than the time when both of the switch elements of the semiconductor switch are turned on, a turning-off section for turning off both of the switch elements of the semiconductor switch a predetermined period later than the time when the other bypass switch is closed, and a switch circuit-changing section for switching the circuit-changing switch from the other bypass switch side to the one bypass switch side.
In the power supply system switching device of the present invention, the controller may be provided with control circuits separate for each of the switching directions of the power supply systems.
Further, in the power supply system switching device of the present invention, the controller may comprise one control circuit, and a connection-switching circuit for switching an external input-output depending on the switching directions of the power supply systems.
Preferably, in the power supply system switching device of the present invention, the current/voltage detection section is externally provided with current detection current-transformers for detecting current flowing through the bypass switches, and the first turning-on section turns on one of the switch elements of the semiconductor switch, based on the detection results of the current directions.
Also preferably, in the power supply system switching device of the present invention, the current/voltage detection section is externally provided with voltage detectors for detecting the voltage polarities at the both ends of the respective bypass switches, and the first turning-on section turns on one of the switch elements of the semiconductor switch, based on the detection results of the voltage polarities.
Preferably, in the power supply system switching device of the present invention, the second turning-on section is externally provided with a voltage detector for detecting the voltage across the both-ends of the semiconductor switch, and based on the state of the applied interpole voltage, it is decided whether current through the bypass switch is zero or not.
Also, in the power supply system switching device of the present invention, the semiconductor switch may comprise a pair of thyristor switches connected in inverse-parallel to each other.
According to the present invention, there is provided a method of switching between power supply systems in which power to a load is switched from a first power supply system to a second power supply system, the load being connected to the connection point of first and second mechanical bypass switches connected in series with the first and second power supply systems, respectively, and a semiconductor switch connected in parallel to the second bypass switch, comprises the steps of opening the first bypass and causing a semiconductor switch to conduct whereby current through the first bypass switch is commutated to flow through the semiconductor switch, and closing the second bypass switch, whereby the load is switched to be connected to the second power supply system.
Preferably, the method of switching between power supply systems, wherein the semiconductor switch comprises a pair of semiconductor switch elements connected in inverse-parallel to each other, comprises the steps of opening the first bypass switch, turning on one of the switch elements of the semiconductor switch based on the current direction or voltage-polarity of current flowing through the first bypass switch, turning on both of the switch elements of the semiconductor switch a predetermined period later then the time when the current through the first bypass switch becomes zero, closing the second bypass switch a predetermined period later than the time when both of the switch elements of the semiconductor switch are turned on, and turning off both of the switch elements of the semiconductor switch a predetermined period later than the time when the second bypass switch is closed.
Also preferably, the method of transferring between the first and second power supply systems in both directions, wherein a circuit-changing switch for circuit-changing the semiconductor switch to connect selectively in parallel to either one of the first and second bypass switches is further provided, and wherein the semiconductor switch comprises a pair of semiconductor switch elements connected in inverse-parallel to each other, comprises the steps of opening one of the first and second bypass switches, turning on one of the switch elements of the semiconductor switch, based on the current directions or voltage polarities of currents flowing through the first and second bypass switches, turning on both of the switch elements of the semiconductor switch a predetermined period later than the time when current flowing through said one bypass switch becomes zero, closing the other bypass switch a predetermined period later than the time when both of the switch elements of the semiconductor switch are turned on, turning off both of the switch elements of the semiconductor switch a predetermined period later than the time when the other bypass switch is closed, and switching the circuit-changing switch from the other bypass switch side to the one bypass switch side.
As the bypass switch employed in the static different systems switching device according to the present invention, a switch which uses a spring operation system to realize a high opening-characteristic, that is, an opening time=abut 5 ms is applied. On the other hand, for turning-on of this switch, a relatively long time, that is, about 50 ms, is required, due to the mechanical restrictions. Therefore, turning-on of the bypass switches is carried out by means of the thyristor switches connected in inverse-parallel to each other so that the turning-on can be performed without 2 ms. By this combination, two different power supply systems can be switched at a high speed of up to xc2xd cycle. In addition, switching between the two power supply systems in both directions can be performed by using one thyristor switch in common. That is, a very economical device can be realized.
In the power supply system switching device of the present invention, under normal conduction, the thyristors are off, and current flows through the bypass switch side. Therefore, the steady conduction loss of the thyristors can be reduced to zero, and moreover, switching of the two different power supply systems in both directions can be performed by using one thyristor switch in common. Accordingly, a power supply system switching device and a method of switching between the power supply systems presenting a low loss and being economical can be provided.